Direct part marking (DPM) allows workpieces to be directly marked, identified and traced to their origin, and its use is growing in the automotive, aerospace, electronics, medical equipment, tooling, and metalworking industries, among many others. Despite the ability to control very tight specifications on element size, width, spacing and so on, the lack of sharp contrast of machine-readable optical DPM codes directly marked on metal, plastic, leather, glass, etc., workpieces prevents traditional moving laser beam readers from electro-optically reading the DPM codes reliably. These moving beam readers emit a laser beam, which reflects off the highly reflective, typically non-planar, metal or glass, workpieces as bright light.
To counter a variety of problems, such as lack of contrast, difficulty of maintaining precise element specifications, limited available marking areas, and a large amount of data to be encoded, the art proposed the use of matrix codes, especially the DataMatrix code, which reduces the required marking element size, precision and area, as well as contrast so that markings are able to be directly made on parts with, for example, steel or aluminum surfaces, and also proposed the use of imaging readers, for example, as disclosed in U.S. Pat. No. 7,201,321, which use solid-state arrays or imagers similar to those used in digital cameras to capture an image of the marking. A microprocessor is used to analyze and decode the captured image of the matrix code.
Yet, the use of imaging readers, especially handheld readers, for reading marked workpieces has proven to be challenging. Contrast is still often less than desirable. Ambient lighting conditions are variable. Illumination from on-board illuminators or illumination light sources is directed at variable angles. Reflections from ambient light sources and illumination light sources often appear in the field of view of the reader as hot spots, glare, or specular reflections of intense, bright light that saturate the imagers, thereby degrading reading performance. Unlike machine-readable codes printed in one color (for example, black) on paper of another color (for example, white), DPM codes are typically difficult for a human operator to even find on the workpieces, which often have complicated, i.e., non-planar, shapes to further complicate finding the DPM code and aiming the reader directly at the DPM code for reading.
Bulk diffusers are commonly used to evenly spread and diffuse illumination light to minimize such hot spots, glare, and specular reflections. However, the level of diffusion is inversely proportional to light transmission. Hence, it would be extremely inefficient to only use highly diffusive material. Moreover, since the illumination light sources are usually placed close to the diffuser, it is difficult to eliminate hot spots on the diffuser. The hot spots become significantly worse when decoding DPM codes on a reflective, curved surface. Therefore, it is desirable and yet challenging to eliminate hot spots while maintaining maximum light throughput and providing uniform background illumination for the DPM code of interest.
In U.S. Pat. No. 6,341,878, a bulk diffuser is used in conjunction with a rear-diffused reflector to eliminate hot spots in a DPM imaging reader. The main disadvantage with this reader is that it requires extremely bright light sources and has a low light throughput. Another shortcoming is that since all the illumination light is reflected off a diffused reflector, a uniform illumination across the diffuser is not provided. In addition, since the light sources are facing toward the imager, an all-enclosed baffling structure is necessary to eliminate stray light.